1. Field of the Invention
The invention is related to the field of drilling and analyzing of core samples from a wellbore penetrating earth formations. More specifically, the invention is related to the use of synthetic sponge liners in a core sample storage chamber for capturing, and preventing loss, of liquid hydrocarbons which may be present in the core sample during transport of the core sample to the earth's surface from the depth within the wellbore at which the sample was drilled.
2. Discussion of the Related Art
Core samples are typically drilled from a rotary drilling rig, in a predetermined earth formation which will eventually be penetrated by a wellbore. The core samples are used to obtain data concerning rock composition, porosity type and volume, and fluid content within the pore space in the formation.
In order to drill the core sample, drilling the wellbore must stop near the top of the predetermined formation, and a drilling assembly is replaced with a coring assembly. The coring assembly typically includes a coring bit, which comprises an annular cylindrical cutting surface. The cutting surface has a hollow center which captures a cylindrical section of the predetermined formation which is formed as a result of the coring bit penetrating the formation. When the hollow center of the coring bit is filled with the core sample, the coring bit is brought to the earth's surface to retrieve the core sample for analysis.
The wellbore is typically drilled using a fluid called drilling mud, which is used to maintain hydrostatic pressure against a fluid pressure which can be present in the pore space of certain earth formations. The mud also maintains the mechanical stability of the open wellbore. In order to maintain hydrostatic pressure, the drilling mud can have a density which is equivalent to a fluid having a pressure gradient ranging from 0.5 to as much as 1.2 psi per foot of wellbore depth. As the core sample is brought to the earth's surface from deeper in the wellbore, fluids contained in pore spaces in the core sample can be expelled from the pore spaces by exsolution of gas. Exsolution results from decreasing hydrostatic pressure on the core sample as the core sample is brought to the earth's surface.
In core samples which contain some amount of petroleum, it is very common for gas dissolved in liquid hydrocarbon to come out of solution as the external hydrostatic pressure is reduced, and as a result, liquid hydrocarbon contained in the pore space can be expelled from the pore space. If the core sample had been intended for obtaining information about possible petroleum content, the loss of liquid hydrocarbon could compromise the analysis.
It is known in the art to provide a sponge liner within the coring bit to capture liquid hydrocarbons which may be displaced by exsolution of gas during recovery of the core sample. For example, "DBS Sponge Coring System", Diamant Boart Stratabit, Houston, Tex. 1988, describes a coring bit having an integral polyurethane sponge liner which absorbs liquid hydrocarbons which may be displaced from the pore space in the core sample.
One of the limitations of using the drilling rig to drill the core sample is that the depth of the predetermined formation may not be precisely known before the wellbore is drilled. Therefore it is difficult to determine the precise depth at which to stop drilling and attach the coring assembly.
In geographic areas in which knowledge about the earth formations is limited, it may not be known prior to drilling the wellbore which formations would provide useful data from the core sample. There may also be a plurality of earth formations in a particular wellbore in which core samples could provide valuable information. It is sometimes uneconomical to drill a plurality of core samples by using the drilling rig in a particular wellbore.
It is known in the art to obtain core samples from the wall of the wellbore after the wellbore has been drilled. Typically, the wellbore will be surveyed with at least one well logging instrument to determine, among other things, from which formations a core sample would likely provide useful information from.
An instrument for rotary drilling core samples from the wellbore wall, the core samples so drilled being known as sidewall cores, is known in the art. For example, "The Rotary Sidewall Coring Tool", Atlas Wireline Services, Houston, Tex., 1993, describes an instrument that can drill a plurality of core samples at any depths within the wellbore chosen by the operator.
The instrument known in the art for taking rotary drilled sidewall cores uses a receiving barrel, disposed within the instrument housing, for storing the core samples until the instrument is brought to the earth's surface. The receiving barrel is in hydraulic communication with the wellbore, so that as the instrument is brought to the earth's surface for recovery of the core samples, any liquid hydrocarbon which may be present in the pore space of the core samples can be driven out of the pore space by exsolution of gas, in substantially the same way as in core samples drilled by using the drilling rig. Analysis of the original liquid hydrocarbon content of the core samples could be compromised.
It is an object of the present invention to provide a sidewall coring tool which stores a plurality of core samples in an absorbent sponge liner so that liquid hydrocarbons which may be driven out of the core samples will be captured by the sponge liner for later analysis.